Optical scanning device and cover glass cleaning mechanism for optical scanning device

ABSTRACT

The present invention relates to an optical scanning device in which a light beam is incident twice on a deflective reflecting facet and its object is to reduce curvature in scanning line trail or make the curvature substantially zero, or correct scanning line displacement due to the curvature in scanning line trail. Two stationary plane mirrors ( 13, 14 ) are disposed to face a deflective reflecting facet ( 11 ) which can be rotated about its rotational axis ( 12 ) such that a light beam (a 1 ) being incident on and reflected from the deflective reflecting facet ( 11 ) is reflected by the two stationary plane mirrors ( 13, 14 ) sequentially. The reflected light beam (a 3 ) is incident on and reflected by the deflective reflecting facet ( 11 ) again. Assuming that a plane being parallel to the rotational axis ( 12 ) and including the light beam (a 0 ) which is first incident on the deflective reflecting facet is an incident plane, the central ray of an emergent light beam (a 4 ) when the emergent light beam (a 4 ) after the second reflection by the deflective reflecting facet ( 11 ) is on the incident plane and a straight line as the central ray of a light beam being projected on the incident plane when the deflective reflecting facet ( 11 ) is revolved by the maximum rotational angle are set substantially parallel to each other.

TECHNICAL FIELD

The present invention relates to an optical scanning device to be usedfor exposure for writing image of an image forming apparatus such as alaser beam printer and, more particularly, to an optical scanning devicein which an optical beam is sequentially incident twice on deflectivereflecting facet(s) of a rotating polygonal mirror or the like.

The present invention relates to a mechanism for cleaning a cover glassof an optical scanning device which is capable of securely removing dustadhering to the surface of the cover glass.

BACKGROUND ART

Conventionally, an optical device comprising a deflective reflectingfacet and two stationary plane mirrors has been proposed in JapaneseUnexamined Patent Publication No. S51-6563. The two stationary planemirrors are disposed to face the deflective reflecting facet which canbe rotated or swivelled about its rotational axis. An incident lightbeam deflected by the deflective reflecting facet is reflected by thetwo stationary plane mirrors sequentially, and is incident on thedeflective reflecting facet again, thereby correcting displacement inexit direction of deflected optical beam due to misalignment of therotational axis of the deflective reflecting facet or misalignment ofeach deflective reflecting facet itself.

Further, a light deflective optical system comprising a deflectivereflecting facet and two stationary plane mirrors has been proposed inJapanese Unexamined Patent Publication No. S61-7818 (U.S. Pat. No.4,796,965). The two stationary plane mirrors are disposed to face thedeflective reflecting facet which can be revolved or swivelled about itsrotational axis and the two stationary plane mirrors are arranged suchthat the edge lines of the two mirrors are on a plane perpendicular tosaid rotational axis of the deflective reflecting facet. An incidentlight beam is incident on the deflective reflecting facet through aspace between the two stationary plane mirrors. The deflected light beamreflected by the deflective reflecting facet is reflected by the twostationary plane mirrors sequentially and is incident on the deflectivereflecting facet again so that the deflected light beam reflected isprojected to pass through a space between the deflective reflectingfacet and the two stationary plane mirrors or between the two stationaryplane mirrors, thereby correcting scanning line distortion.

However, since no analytical study has been made and no reference aboutincident angle of 20° or less has been made in the Japanese UnexaminedPatent Publication No. S61-7818, the light deflective optical systemcannot properly correct scanning line distortion.

In the optical system in which light beam deflected twice by the samedeflective reflecting facet, the trail of the light beam may be curvedafter the second time deflecting reflection depending on the incidentangle of the light beam. Therefore, depending on position of the opticalaxis direction of an optical member such as a lens of the scanningoptical system after deflection, a wider effective range in thesub-scanning direction (perpendicular to the scanning direction) isrequired. However, it is hard to manufacture an anamorphic lens, to beused in a scanning optical system, having a wide effective range both inthe scanning direction and the sub scanning direction. This causesanother problem that the degree of freedom of arrangement among aplurality of lenses is reduced.

The narrower the required effective range of optical member such as alens is, the higher the accuracy of an optical face is easily obtained.

In any event, it is very advantageous if the curvature in scanning linetrail is restrained to very small at any position in the opticaldirection.

By the way, in an optical scanning device like the optical scanningdevice proposed in Japanese Unexamined Patent Publication No. S51-6563in which an incident light beam is reflected by even number stationaryplane mirrors sequentially so that the light beam is incident twice onthe same deflective reflecting facet to deflect the light beam, there isno necessary to provide an optical system for correcting the surfacetilt error in the deflective reflecting facet from a viewpoint ofcompensation of scanning line displacement.

However, the scanning angle must be small because the light beaminterferes with the stationary plane mirrors when the light beam isincident on and projected to the deflective reflecting facet at a rightangle to the sub-scanning direction. Therefore, the incident or exitangle is required to be an angle not the right angle to the deflectivereflecting facet. This may make a difference in exit angle that causesthe curvature of scanning line trail.

In addition, a scanning line displacement on an imaging surface may becaused. The amount of the scanning line displacement is a productobtained by multiplying the displacement at the second deflection due tothe surface tilt error by a transversal enlargement ratio β of thesub-scanning direction of the scanning optical system.

In case that the value of surface tilt error varies at the beam incidentposition depending on the rotation of the deflective reflecting facetbecause the deflective reflecting facet has a twisted or curved portion,there must be differences in exit angle as compared to a case that thedeflective reflecting facet has a complete plane, thus causing furthercurvature in the scanning line trail. In case that the twisting orcurving degrees of deflective reflecting facets differ from each other,the exit angles vary every deflective reflecting facet, thus causingscanning line distortion on the imaging surface.

DISCLOSURE OF THE INVENTION

The present invention was made to overcome the aforementioned problemsof conventional techniques. The first object of the present inventionis, in an optical scanning device in which a light beam is incidenttwice on a deflective reflecting facet, to reduce curvature in scanningline trail or make the curvature in scanning line trail to besubstantially zero at a certain position, thereby easing the limitationon arrangement of lenses in the scanning optical system in the opticaldirection.

The second object of the present invention is to provide an opticalscanning device in which an incident light beam is reflected by evennumber stationary plane mirrors sequentially so that the light beam isincident twice on the same deflective reflecting facet to deflect thelight beam and which is characterized in that no curvature in scanningline based on curvature in scanning line trail caused by difference inexit angle is created so that the scanning line displacement due tosurface tilt error or variation in exit angle is corrected.

The third object of the present invention is to provide a cover glasscleaning mechanism for an optical scanning device which is structuredsuch that dust and dirt adhering to the surface of a cover glass can besecurely removed.

A first optical scanning device of the present invention achieving thefirst object mentioned above is an optical scanning device comprising alight deflective optical system in which two stationary plane mirrorsare disposed to face a deflective reflecting facet which is parallel toa rotational axis and can be rotated or swivelled about the rotationalaxis such that a light beam being incident on and reflected from saiddeflective reflecting facet is reflected by said two stationary planemirrors sequentially and is incident on and reflected by the deflectivereflecting facet again, wherein

assuming that a plane being parallel to said rotational axis andincluding the light beam to be first incident on said deflectivereflecting facet is an incident plane, said two stationary plane mirrorsare disposed perpendicular to said incident plane, and

the central ray of an emergent light beam when the emergent light beamafter the second reflection by said deflective reflecting facet is onsaid incident plane and a straight line as the central ray of anemergent light beam being projected on the incident plane when thedeflective reflecting facet is revolved by the maximum rotational angleare set substantially parallel to each other.

In this case, the optical scanning device may be structured such thatthe light beam to be first incident on the deflective reflecting facetpasses through a space between the two stationary plane mirrors and thedeflected light beam after the second reflection by the deflectivereflecting facet passes through the space between the two stationaryplane mirrors and may be structured such that the one of the twostationary plane mirrors is positioned to be sandwiched between thelight beam first incident on the deflective reflecting facet and theemergent light beam after the second reflection by the deflectivereflecting facet.

Further, the optical scanning device is preferably structured to satisfythe following relation:0.33·θ1≦θ2≦0.37·θ1  (21)where θ1 is the incident angle of the central ray of the light beam tobe first incident on the deflective reflecting facet at a position wherethe deflective reflecting facet is perpendicular to the incident planeand θ2 is the exit angle of the central ray of the emergent light beamafter the second reflection by the deflective reflecting facet.

A second optical scanning device of the present invention is an opticalscanning device comprising a light deflective optical system in whichtwo stationary plane mirrors are disposed to face a deflectivereflecting facet which is parallel to a rotational axis and can berotated or swivelled about the rotational axis such that a light beambeing incident on and reflected from said deflective reflecting facet isreflected by said two stationary plane mirrors sequentially and isincident on and reflected by the deflective reflecting facet again,wherein

assuming that a plane being parallel to said rotational axis andincluding the light beam to be first incident on said deflectivereflecting facet is an incident plane, said two stationary plane mirrorsare disposed perpendicular to said incident plane, and

said optical scanning device is structured to satisfy the followingrelation:0.33·θ1-1.27≦θ2≦0.35·θ1-1.50  (22)where θ1 is the incident angle of the central ray of the light beam tobe first incident on said deflective reflecting facet at a positionwhere said deflective reflecting facet is perpendicular to the saidincident plane and θ2 is the exit angle of the central ray of theemergent light beam after the second reflection by said deflectivereflecting facet.

Also in this case, the optical scanning device may be structured suchthat the light beam to be first incident on the deflective reflectingfacet passes through a space between the two stationary plane mirrorsand the deflected light beam after the second reflection by thedeflective reflecting facet passes through the space between the twostationary plane mirrors and may be structured such that the one of thetwo stationary plane mirrors is positioned to be sandwiched between thelight beam first incident on the deflective reflecting facet and theemergent light beam after the second reflection by the deflectivereflecting facet.

A third optical scanning device of the present invention is an opticalscanning device comprising a light deflective optical system in whichtwo stationary plane mirrors are disposed to face a deflectivereflecting facet which is parallel to a rotational axis and can berotated or swivelled about the rotational axis such that a light beambeing incident on and reflected from said deflective reflecting facet isreflected by said two stationary plane mirrors sequentially and isincident on and reflected by the deflective reflecting facet again,wherein

assuming that a plane being parallel to said rotational axis andincluding the light beam to be first incident on said deflectivereflecting facet is an incident plane, said two stationary plane mirrorsare disposed perpendicular to said incident plane, and

the central ray of an emergent light beam when the emergent light beamafter the second reflection by said deflective reflecting facet is onsaid incident plane and a straight line as the central ray of anemergent light beam being projected on the incident plane when thedeflective reflecting facet is revolved by the maximum rotational angleintersect with each other at a position near at least one of opticalsurfaces disposed in the optical axial direction of a scanning opticalsystem between said deflective reflecting facet and an imaging surface.

Also in this case, the optical scanning device may be structured suchthat the light beam to be first incident on the deflective reflectingfacet passes through a space between the two stationary plane mirrorsand the deflected light beam after the second reflection by thedeflective reflecting facet passes through the space between the twostationary plane mirrors and may be structured such that the one of thetwo stationary plane mirrors is positioned to be sandwiched between thelight beam first incident on the deflective reflecting facet and theemergent light beam after the second reflection by the deflectivereflecting facet.

A fourth optical scanning device of the present invention achieving thesecond object mentioned above is an optical scanning device comprising:a light deflective optical system in which two stationary plane mirrorsare disposed to face a deflective reflecting facet which is parallel toa rotational axis and can be rotated or swivelled about the rotationalaxis such that a light beam being incident on and reflected from saiddeflective reflecting facet is reflected by said two stationary planemirrors sequentially and is incident on and reflected by the deflectivereflecting facet again; an illumination optical system which irradiatesa light beam to said deflective reflecting facet; and a scanning opticalsystem which projects the light beam deflected by said light deflectiveoptical system to an imaging surface to form a scanning line, wherein

assuming that a plane being parallel to said rotational axis andincluding the light beam to be first incident on said deflectivereflecting facet is an incident plane, said two stationary plane mirrorsare disposed perpendicular to said incident plane, and

an emergent light beam when the emergent light beam after the secondreflection by said deflective reflecting facet is on said incident planehas such a relation that the second reflection point and said imagingsurface are substantially conjugated to each other on the incidentplane.

In this case, the optical scanning device is preferably structured suchthat the light beam to be first incident on the deflective reflectingfacet passes through a space between the two stationary plane mirrorsand the deflected light beam after the second reflection by thedeflective reflecting facet passes through the space between the twostationary plane mirrors.

It is also preferable that, when the emergent light beam after thesecond reflection by the deflective reflecting facet is on the incidentplane, the light beam irradiated from the illumination optical systemconverges near the second reflection point on the incident plane.

Further, it is preferable that the following relation is satisfied:|β·Ld·tan (2ε)/cos (2ε)|≦0.25·LP  (30)or the following relation is satisfied:|β·Ld·tan (2ε)/cos (2ε)|≦0.125·LP  (31)where Ld is the transfer distance from the first deflection point to thesecond deflection point when the emergent light beam after the secondreflection by the deflective reflecting facet is on the incident plane,ε is the angle of a surface tilt error in the deflective reflectingfacet, β is the transverse magnification in the direction of theincident plane of the scanning optical system, and LP is a scanningpitch in the imaging surface.

For example, the optical scanning device of the present invention asmentioned above is preferably employed for exposure for writing image inan image forming apparatus.

According to the first through third optical scanning devices of thepresent invention, an optical scanning device employing a lightdeflective optical system of twice incidence type comprising adeflective reflecting facet and two stationary plane mirrors ischaracterized in that the central ray of an emergent light beam when theemergent light beam after the second reflection by the deflectivereflecting facet is on the incident plane and a straight line as thecentral ray of an emergent light beam being projected on the incidentplane when the deflective reflecting facet is revolved by the maximumrotational angle are set substantially parallel to each other, theequation (22) is satisfied, or the central ray of an emergent light beamwhen the emergent light beam after the second reflection by thedeflective reflecting facet is on the incident plane and a straight lineas the central ray of an emergent light beam being projected on theincident plane when the deflective reflecting facet is revolved by themaximum rotational angle intersect with each other at a position near atleast one of optical surfaces disposed in the optical axial direction ofa scanning optical system. Therefore, curvature in scanning line trailon the optical surface(s) in the scanning optical system is reduced orbecomes substantially zero, thereby increasing the degree of freedom ofposition arrangement of the scanning optical system and allowing thereduction in dimension in the sub scanning direction of a scanningoptical system, thus achieving an inexpensive scanning optical devicewhich is small and has high accuracy.

According to the fourth optical scanning device of the presentinvention, an optical scanning device employing a light deflectiveoptical system of twice incidence type comprising a deflectivereflecting facet and two stationary plane mirrors is characterized inthat an emergent light beam when the emergent light beam after thesecond reflection by said deflective reflecting facet is on the incidentplane has such a relation that the second reflection point and theimaging surface are substantially conjugated to each other on theincident plane, whereby the position in the sub scanning direction ofthe emergent light beam converging on the imaging surface does not moveeven with variation of rotational angle of the deflective reflectingfacet. Therefore, even when the scanning line trail is curved due to thedifference in exit angle, no curvature in scanning line is caused so asto create a substantially straight scanning line on the imaging surface.

A cover glass cleaning mechanism for an optical scanning device of thepresent invention is a cover glass cleaning mechanism comprising: anoptical system for optical scanning; an imaging surface; a cover glassfor permitting the transmission of a scanning light beam from saidoptical system to said imaging surface; a mounting member to which saidcover glass is mounted; and a cleaning lever which is provided at itsend with a cleaning member for wiping the surface of said cover glass,wherein said cover glass is disposed to project on a mounting member,and the stroke length of said cleaning lever is set such that saidcleaning member can be moved to a position out of the end of the coverglass on at least one of ends in the operational direction of saidcleaning lever.

The present invention is also characterized in that the stroke length ofsaid cleaning lever is set to be longer than the length in thelongitudinal direction of said cover glass.

The present invention also provides a cover glass cleaning mechanism foran optical scanning device comprising: an optical system for opticalscanning; an imaging surface; a cover glass for permitting thetransmission of a scanning light beam from said optical system to saidimaging surface; a mounting member to which said cover glass is mounted;and a cleaning lever which is provided at its end with a cleaning memberfor wiping the surface of said cover glass, wherein a guide member isprovided on said cover glass so that said cleaning lever is reciprocatedalong said guide member and parallel to said cover glass.

The present invention is characterized in that said guide member isarranged to extend over the entire length of said cover glass.

The present invention also provides a cover glass cleaning mechanism foran optical scanning device comprising: an optical system for opticalscanning; an imaging surface; a cover glass for permitting thetransmission of a scanning light beam from said optical system to saidimaging surface; a mounting member to which said cover glass is mounted;and a cleaning lever which is provided at its end with a cleaning memberfor wiping the surface of said cover glass, wherein said cleaning leveris provided with a slot which is long in the longitudinal direction notto disturb the transmission of scanning beams even when said cleaninglever is fully retracted inside.

The present invention also provides a cover glass cleaning mechanism foran optical scanning device comprising: an optical system for opticalscanning; an imaging surface; a cover glass for permitting thetransmission of a scanning light beam from said optical system to saidimaging surface; a mounting member to which said cover glass is mounted;and a cleaning lever which is provided at its end with a cleaning memberfor wiping the surface of said cover glass, wherein a stopping member isprovided for stopping the further movement of said cleaning lever at theterminal end when said cleaning lever is fully retracted inside.

Further, the present invention is characterized in that said stoppingmember is formed in said mounting member for said cover glass.

Furthermore, the present invention is characterized in that saidstopping member is formed in said guide member.

The present invention also provides a cover glass cleaning mechanism foran optical scanning device comprising: an optical system for opticalscanning; an imaging surface; a cover glass for permitting thetransmission of a scanning light beam from said optical system to saidimaging surface; a mounting member to which said cover glass is mounted;and a cleaning lever which is provided at its end with a cleaning memberfor wiping the surface of said cover glass, wherein a movementrestriction means is provided for limiting the movement of said cleaninglever during retracting said cleaning lever inside and an engaging meansis provided for engaging said movement restriction means.

Further, the present invention is characterized in that said movementrestriction means is formed in said cleaning lever and said engagingmeans is formed in said mounting member for said cover glass.

The present invention is also characterized in that said movementrestriction means is formed in said cleaning lever and said engagingmeans is formed in said guide member.

Further, the present invention is characterized in that said movementrestriction means is formed in said mounting member for said cover glassand said engaging means is formed in said cleaning lever.

Furthermore, the present invention is characterized in that saidmovement restriction means is formed in said guide member and saidengaging means is formed in said cleaning lever.

The present invention also provides a cover glass cleaning mechanism foran optical scanning device comprising: an optical system for opticalscanning; an imaging surface; a cover glass for permitting thetransmission of a scanning light beam from said optical system to saidimaging surface; a mounting member to which said cover glass is mounted;and a cleaning lever which is provided at its end with a cleaning memberfor wiping the surface of said cover glass, wherein a stopper isprovided for preventing the cleaning lever from coming off said guidemember when said cleaning lever is withdrawn.

The present invention is characterized in that a means for cancelingsaid stopper is provided to allow the removal of the cleaning lever fromsaid guide member so that the cleaning lever is detachable from anapparatus body.

The present invention also provides a cover glass cleaning mechanism foran optical scanning device comprising: an optical system for opticalscanning; an imaging surface; a cover glass for permitting thetransmission of a scanning light beam from said optical system to saidimaging surface; a mounting member to which said cover glass is mounted;and a cleaning lever which is provided at its end with a cleaning memberfor wiping the surface of said cover glass, wherein said cleaning memberis detachably attached to the end of said cleaning lever.

Moreover, the present invention provides a cover glass cleaningmechanism for an optical scanning device comprising: an optical systemfor optical scanning; an imaging surface, a cover glass for permittingthe transmission of a scanning light beam from said optical system tosaid imaging surface; a mounting member to which said cover glass ismounted; and a cleaning lever which is provided at its end with acleaning member for wiping the surface of said cover glass, wherein anelastic member is provided to press said cleaning member against thecover glass.

The cover glass cleaning mechanism of an optical scanning deviceaccording to the present invention comprises the cover glass disposed toproject on the mounting member and a cleaning lever which is provided atits end with a cleaning member for wiping the surface of said coverglass and is structured that the stroke length of said cleaning lever isset such that said cleaning member can be moved to a position out of theend of the cover glass on at least one of ends in the operationaldirection of said cleaning lever. Therefore, when the cleaning lever isretracted fully inside, the cleaning member is out of the cover glassand thus moves to a level lower than the level of the surface of thecover glass so that the dist and dirt attached to the cleaning memberare shaken off, thereby preventing dist and dirt remaining on thecleaning member from adhering to the cover glass again when the cleaninglever is returned. Therefore, even with repeated cleaning operation, thecleaning member and the surface of the cover glass can be maintained inthe cleaned state.

Also in the present invention, the guide member is provided on the coverglass so that the cleaning lever is reciprocated along the guide memberand parallel to the cover glass. Therefore, the cleaning member ispressed against the cover glass with constant pressure, thereby properlycleaning the cover glass uniformly without remaining non-cleanedportions. Further, the guide member prevents the cover glass from beingcontaminated by hands of an operator or adhesion of foreign matter.

The guide member extends over the entire length of the cover glass.Therefore, the cleaning member can be prevented from coming off thecleaning lever and thus prevented from being contaminated by touchingportions other than the cover glass. By the existence of the guidemember, the accuracy of position between the cleaning member and thecover glass is improved.

The cleaning lever is provided with the slot which is long in thelongitudinal direction not to disturb the transmission of scanning beamseven when the cleaning lever is fully retracted inside. Since theoptical scanning device is therefore always available even in a statethat the cleaning lever is attached, there is no need to detach andstore the cleaning lever, thereby preventing the cleaning lever frombeing missing and further preventing a storing place from beingcontaminated by the cleaning lever.

The stopping member is provided for stopping the further movement of thecleaning lever at the terminal end when the cleaning lever is fullyretracted inside. Therefore, the excessive movement of the cleaninglever is prevented, thereby achieving the economical movement of thecleaning lever.

The movement restriction means is provided for limiting the movement ofthe cleaning lever during retracting the cleaning lever inside and anengaging means is provided for engaging the movement restriction means.This structure can prevent such an event that dust and dirt attached tothe cleaning member fly to the surface of the cover glass due tovibration of the cleaning lever. In addition, the engaging meanscomposed of the concave and convex portions as mentioned above alsofunctions as a click mechanism so that an operator can sensuously checkwhen the cleaning lever reaches the traveling terminal position duringpushing the cleaning lever. It should be noted that the number or partscan be reduced by integrally forming the engaging means to the mountingmember.

The stopper is provided for preventing the cleaning lever from comingoff the guide member when the cleaning lever is withdrawn. Thisstructure eliminates the need of aligning the cleaning lever to theguide member for the operation of pushing the cleaning lever again, thusincreasing the operability. This structure also prevents the cleaninglever from being missing.

The means for canceling the stopper is provided to allow the removal ofthe cleaning lever from the guide member so that the cleaning lever isdetachable from an apparatus body. Therefore, the convenience inmaintenance such as cleaning of the cleaning member and the replacementof the cleaning member according to deterioration is improved.

Further, the cleaning member is detachably attached to the end of thecleaning lever. Therefore, the inspection and the replacement of thecleaning member can be easily conducted.

Furthermore, since the elastic member is provided to press the cleaningmember against the cover glass, dust and dirt adhering to the surface ofthe cover glass can be effectively removed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the entire structure of an opticalscanning device according to a first embodiment of the presentinvention;

FIG. 2 is a perspective view showing the light deflective optical systemas main parts of the optical scanning device shown in FIG. 1;

FIG. 3 is an illustration for explaining the curvature in scanning linetrail of light beam after the first reflection at a deflectivereflecting facet;

FIG. 4 is an illustration for explaining the definition of a relationbetween an incident angle θ1 of an incident light beam relative to adeflective reflecting facet and an exit angle θ2 of an emergent lightbeam;

FIG. 5 is an illustration showing a state that light beams are projectedon an incident plane;

FIG. 6 is an illustration showing a state that the light beams areprojected on a plane perpendicular to the rotational axis of thedeflective reflecting facets;

FIGS. 7( a), 7(b) are illustrations showing angular relations betweenlight beams a0 and a1 when the deflective reflecting facet facesforthright and when the deflective reflecting facet is revolved by arotational angle ω;

FIG. 8 is an illustration showing an angular relation when thedeflective reflecting facet for light beams a3 and a4 is revolved by arotational angle ω;

FIG. 9 is an illustration for studying components of the directionalvector of the light beam a1 after the first reflection at the deflectivereflecting facet;

FIG. 10 is an illustration for studying components of the directionalvector of the light beam a3 to be incident again on the deflectivereflecting facet;

FIG. 11 is a graph plotting the relation between θ1 and θ2 whichsatisfies Φ2=0;

FIG. 12 is a graph showing a boundary of a range satisfying Φ≦0.6°;

FIGS. 13( a), 13(b) are illustrations for explaining a sag amount Δ1 atthe first deflective reflection point and a sag amount Δ2 at the seconddeflective reflection point according to the revolution of thedeflective reflecting facets;

FIGS. 14( a), 14(b) are an illustration for explaining Δ′ which is thesag amount Δ1 at the first deflective reflection point plus the sagamount Δ2 at the second deflective reflection point according to therevolution of the deflective reflecting facets, and an enlarged view ofa portion near the second deflective reflection point;

FIGS. 15( a), 15(b) are illustrations showing shift amounts betweenreflected light beams a1 and a4 based on a sag amount Δ1 at the firstdeflective reflection point and a sag amount Δ2 at the second deflectivereflection point;

FIG. 16 is a graph showing the relation between θ1 and θ2 where thecurvature δ is substantially 0 in the first example of the firstembodiment;

FIG. 17 is an illustration of a light deflective optical system as mainparts of an optical scanning device according to a variation of thefirst embodiment of the present invention and showing the angularrelation between light beams a1 and a4 when the deflective reflectingfacet faces forthright;

FIG. 18 is a perspective view showing the entire structure of an opticalscanning device according to a second embodiment of the presentinvention;

FIG. 19 is a perspective view showing the light deflective opticalsystem as main parts of the optical scanning device shown in FIG. 18;

FIG. 20 is a graph plotting differences in exit angle Φ2 relative to therotational angle ω of the deflective reflecting facet caused by changingthe ratio between θ1 and θ2;

FIGS. 21( a), 21(b) are illustrations for explaining the opticalscanning device according to the second embodiment of the presentinvention and showing light beams a4 from a deflective reflecting facetto an imaging surface through the scanning optical system, the lightbeams a4 being illustrated in a state projected on an incident plane;

FIGS. 22( a), 22(b) are auxiliary illustrations similar to FIG. 5 butshowing a case where surface tilt error is caused on the deflectivereflecting facet;

FIG. 23 is an illustration for explaining the movement of scanning lineon the imaging surface when there is a movement at the second deflectionpoint;

FIG. 24 is an illustration showing a state of an emergent light beam a4when the deflective reflecting facet has a twisted or curved portion;

FIGS. 25( a), 25(b) are illustrations for explaining that the thicknessof a polygon mirror can be reduced according to the optical scanningdevice according to the second embodiment of the present invention;

FIG. 26 is an explanatory illustration showing an example of the opticalscanning device;

FIG. 27 is a schematic perspective view showing an example of a coverglass cleaning mechanism for the optical scanning device according tothe present invention;

FIG. 28 is a perspective view showing a state that a cleaning levershown in FIG. 27 is withdrawn;

FIG. 29 is a perspective view showing a state that the cleaning levershown in FIG. 27 is retracted;

FIGS. 30( a), 30(b) are sectional front views partially andschematically showing the cover glass cleaning mechanism;

FIG. 31 is a perspective view schematically showing an embodiment of thepresent invention;

FIG. 32 is a cross sectional view schematically showing an example of aguide member;

FIG. 33 is a cross sectional view schematically showing another exampleof the guide member;

FIG. 34 is a perspective view schematically showing a cleaning leveraccording to the embodiment of the present invention;

FIG. 35 is a perspective view partially and schematically showing oneend in the longitudinal direction of the guide member;

FIG. 36 is a perspective view partially and schematically showing oneend in the longitudinal direction of the cleaning lever;

FIG. 37 is a vertical sectional front view partially showing anembodiment of the present invention;

FIG. 38 is a vertical sectional front view partially showing anembodiment of the present invention;

FIG. 39 is a perspective view schematically showing an embodiment of thepresent invention;

FIG. 40 is a vertical sectional front view schematically showing theembodiment shown in FIG. 39;

FIG. 41 is a perspective view showing the details of a stopper;

FIG. 42 is a vertical sectional front view schematically showing theaction of the stopper;

FIG. 43 is an exploded perspective view of a cleaning lever;

FIG. 44 is a schematic perspective view of the cleaning lever taken frombelow; and

FIG. 45 is a perspective view schematically showing an example of aportion where a conventional cover glass is installed.

BEST MODE FOR CARRYING OUT THE INVENTION

The principle and embodiments of an optical scanning device of thepresent invention will now be described in detail with reference to theaccompanying drawings.

FIG. 1 is a perspective view showing the entire structure of an opticalscanning device according to a first embodiment of the present inventionand FIG. 2 is a perspective view showing the light deflective opticalsystem as main parts of the optical scanning device.

According to this structure, an optical deflector is composed of apolygon mirror 10 taking a form of a polygonal column and having aplurality (six in the illustrated example) of deflective reflectingfacets 11 on the peripheral sides of the polygonal column. The polygonmirror 10 rotates about its rotational axis 12 so that the deflectivereflecting facets 11 revolve about the rotational axis 12. Twostationary plane mirrors 13, 14 are disposed to face a deflectivereflecting facet 11 related to optical deflection so that these planemirrors 13, 14 have an angle relative to each other and have a space 15therebetween.

A light beam from a light source 21 is converted into a parallel lightbeam a0 by a lens 22 and is incident on the deflective reflecting facet11 diagonally from below, in case of drawing taken along the rotationalaxis 12, through the space 15 between the stationary plane mirrors 13and 14. The light beam a0 becomes a light beam a1 after the firstreflection by the deflective reflecting facet 11. The light beam a1proceeds diagonally upward to be incident on one stationary plane mirror13. The light beam a1 becomes a light beam a2 after reflection at thestationary plane mirror 13. The light beam a2 proceeds downward to beincident on the other stationary plane mirror 14. The light beam a2becomes a light beam a3 after reflection at the stationary plane mirror14. The light beam a3 is incident on the deflective reflecting facet 11again. The light beam a3 becomes a light beam a4 after the secondreflection by the deflective reflecting facet 11. The light beam a4proceeds diagonally upward through the space 15 between the stationaryplane mirrors 13 and 14 and is converted into a focused light beam via ascanning optical system 23 and is focused to be incident on an imagingsurface 24. Since the deflective reflecting facets 11 revolve about therotational axis 12, the focused light beam moves at a rotational speedabout four times as faster as the rotational speed of the deflectivereflecting facets 11 to write a scan line b on the imaging surface 24.Adjacent deflective reflecting facets 11 successively come in and goaway the position of incidence of the incident light beam a0 because ofthe rotation of the polygon mirror 10. As a result, according to therotation of the polygon mirror 10, the scan lines b are successivelywritten from one end to the other end on the imaging surface 24 at thesame level. The scanning in this direction is called the main scanning.Sub scanning is conducted by moving a scanned substrate on the imagingsurface 24 in a direction perpendicular to that of the main scanning ata constant rate. The main scanning and the sub scanning cooperate toachieve raster scanning in which scan lines b are aligned with aconstant pitch on the scanned substrate.

The two stationary plane mirrors 13, 14 are disposed perpendicularly tothe incident plane on the condition that the incident plane is definedas a plane being parallel to the rotational axis 12 and including thecentral ray of the incident light beam a0.

FIG. 3 is an illustration for explaining the curvature of the scanningline trail b′ of the light beam a1 after the first reflection at thedeflective reflecting facet 11. When the incident light beam a0 is notparallel to the rotational plane (plane perpendicular to the rotationalaxis 12) of the deflective reflecting facets 11, the scanning line trailb′ of the light beam a1 reflected at the deflective reflecting facets 11is curved as shown in FIG. 3. In FIG. 3, the degree of the curvature δof the scanning line trail b′ is defined by an amount shifting from thescanning line trail b″ without curvature in a plane perpendicular to thelight beam a1 on the incident plane. The difference between the scanningline b and the scanning line trail b′, b″ will be described. That is,the scanning line b is a scanning line trail of a beam collected ontothe imaging surface 24 via a scanning optical system 23, while thescanning line trail is a trail of an emergent light beam a4 at anyposition from the deflective reflecting facet 11 to the imaging surface24.

In the light deflective optical system of twice incidence type shown inFIG. 1 and FIG. 2, even when the scanning optical system 23 is adaptednot to cause curvature in scanning line b due to angles θ1, θ2 (FIG. 4)of the incident light beam a0 and the emergent light beam a4, curvaturein scanning line trail should be caused. The definition of angles θ1, θ2will be described later.

In the deflective optical system of twice incidence type, curvature inscanning line trail is caused even between the first deflectivereflection and the second deflective reflection as apparent from FIG. 3.However, light path length between the first deflection and the seconddeflection is fairly shorter than the light path length of the entirescanning optical apparatus (light path length from the first reflectionpoint on the deflective reflecting facet 11 to the imaging surface 24)so that the degree of curvature caused between the first deflection andthe second deflection is small. The curvature in scanning line trailcaused according to the distance from the deflective reflecting facet 11is rather dominant because of a difference in exit angle of the emergentlight beam a4 according to the deflection in the light deflectiveoptical system.

Hereinafter, the difference in exit angle caused by the deflection ofthe emergent light beam a4 in the structure as shown in FIG. 1 and FIG.2 will be studied.

As shown in FIG. 4, when the deflective reflecting facet 11 facesforthright, i.e. perpendicularly to the incident plane, all of the lightbeams a0 through a4 are on a plane of paper of the figure. It is assumedthat the incident angle of the incident light beam a0 relative to thedeflective reflecting facet 11 is θ1 and the exit angle of the emergentlight beam a4 is θ2.

FIG. 5 is an illustration showing a state that the light beams a0through a4 are projected on the incident plane. In this figure, solidlines indicate the state when the deflective reflecting facet 11 facesforthright so that the light beams a0 through a4 are on the incidentplane and broken lines indicate the state that the light beams a0through a4 are projected on the incident plane when the deflectivereflecting facet 11 is revolved by a rotational angle ω from theposition where the deflective reflecting facet 11 faces forthright. FIG.6 is an illustration showing that the light beams a0 through a4 areprojected on a plane (rotation plane) perpendicular to the rotationalaxis 12 of the deflective reflecting facet 11 when the deflectivereflecting facet 11 is revolved by a rotational angle ω from theposition where the deflective reflecting facet 11 faces forthright.

FIGS. 7( a), 7(b) are illustrations showing angular relations betweenthe light beams a0 and a1, wherein FIG. 7( a) shows the angular relationbetween the light beams a0 and a1 when the deflective reflecting facet11 faces forthright and FIG. 7( b) shows the angular relation betweenthe light beams a0 and a1 when the deflective reflecting facet 11 isrevolved by a rotational angle ω. FIG. 8 is an illustration showing anangular relation when the deflective reflecting facet for light beams a3and a4 is revolved by a rotational angle ω. In these figures, as shownin FIG. 5, φ1 indicates a difference in exit angle of the light beam a1,as projected on the incident plane, between the exit angle when thedeflective reflecting facet 11 faces forthright and the exit angle whenthe deflective reflecting facet 11 is revolved by the rotational angleω, and φ2 indicates a difference in exit angle of the light beam a4, asprojected on the incident plane, between the exit angle when thedeflective reflecting facet 11 faces forthright and the exit angle whenthe deflective reflecting facet 11 is revolved by the rotational angleω. In addition, 2ω′ indicates an angle of the light beam a3 relative tothe incident plane as projected on a plane perpendicular to therotational axis 12 of the deflective reflecting facet 11 when thedeflective reflecting facet 11 is revolved by the rotational angle ω asshown in FIG. 6.

As mentioned above, the difference in exit angle caused by thedeflection of the emergent light beam a4 is an angle φ2 formed betweenthe emergent light beam a4 when the emergent light beam a4 is on theincident plane and a straight line as the emergent light beam a4 beingprojected on the incident plane when the deflective reflecting facet 11is revolved by the maximum rotational angle ω in FIG. 5 which is aprojection drawing showing a state that all light beams being projectedon the incident plane.

As the scanning optical system 23 of the light scanning device, anoptical member as a molded plastic or the like which is long in thescanning direction of the light beam a4 is generally used. In theoptical member of a molded plastic, it is difficult to have an effectiverange of more than about 5 mm in the transverse direction. Therefore, itis desired to limit the curvature of scanning line trail at the positionof the scanning optical system 23 to about 5 mm or less.

The curvature of scanning line trail can be limited within the lenseffective range of the scanning optical system 23 by making the straightline as the central ray of the emergent light beam a4 being projected onthe incident plane when the deflective reflecting facet 11 is revolvedby the maximum rotational angle ω to be substantially parallel to thecentral ray of the emergent light beam a4 when the emergent light beama4 is on the incident plane (that is, making the emergent light beam a4indicated by a broken line to be substantially parallel to the emergentlight beam a4 indicated by a solid line in FIG. 5). The aforementioned“substantially parallel” state is in such a range that the curvature inscanning line trail on the lens composing the scanning optical system 23can be limited within 5 mm, this value being decided according to thelimitation of the effective range in the transverse direction of theoptical member composed of a molded plastic. It should be noted thatcentral ray means light ray passed through the optical axis of the lens22 composing the illumination optical system. The light beams a0 througha4 as described later mean such light rays.

Here, the aforementioned difference in exit angle φ2 will be studied.

First, the first deflection will be considered. As apparent from FIG. 7(b), φ1 is given by:L·tan (θ1+φ1)=L·tan φ1/cos 2ω  (1)∴φ1=tan⁻¹ (tan θ1/cos 2ω)−θ1  (2)

Now, the light beam a3 to be incident again upon the deflectivereflecting facet 11 will be considered.

The following explanation will be made using a coordinate systemcomprising coordinate axes S, T, and U as shown in FIG. 9, in which theincident plane corresponds to an S-U plane and the plane perpendicularto the rotational axis 12 of the deflective reflecting facets 11corresponds to a T-U plane. Assuming that components in the axialdirections S, T, and U of the directional vector (unit vector) of thelight beam a1 after the first reflection at the deflective reflectingfacet 11 are represented by (s, t, u), respectively, the followingequation is obtained:√(s ² +t ² +u ²)=1  (3)The magnitude d of the directional vector of the light beam a1 asprojected on the S-U plane is given by:d=√(s ² +u ²)  (4)

Because the rotational angle of the deflective reflecting facet 11 is ω,the following equations are obtained:u·tan 2ω=t  (5)u·tan (θ1+φ1)=s  (6)

From the equations (3), (5), and (6), the following equation can beobtained:u ² =1/{tan² (θ1+φ1)+tan² 2ω+1}  (7)From the equations (4), (6), and (7), the following equation can beobtained:

$\begin{matrix}\begin{matrix}{d^{2} = {u^{2} \cdot \left\{ {{\tan^{2}\left( {{\theta\; 1} + {\phi\; 1}} \right)} + 1} \right\}}} \\{= {\left\{ {{\tan^{2}\left( {{\theta\; 1} + {\phi\; 1}} \right)} + 1} \right\}/\left\{ {{\tan^{2}\left( {{\theta\; 1} + {\phi\; 1}} \right)} + {\tan^{2}2\;\omega} + 1} \right\}}}\end{matrix} & (8)\end{matrix}$

Also as for FIG. 10, using a coordinate system similar to that of FIG. 9and assuming that components in the axial directions S, T, and U of thedirectional vector (unit vector) of the light beam a3 to be incidentagain on the deflective reflecting facet 11 are represented by (s′, t′,u′), respectively, the following equation is obtained:√(s′ ² +t′ ² +u′ ²)=1  (9)Since the light beam is reflected at the stationary plane mirrors 13 and14 perpendicular to the S-U plane (incident plane) after the firstdeflection before the second deflection, the magnitude d′ of (s′, t′,u′) as projected on the S-U plane (the magnitude d′ of the directionalvector of the light beam a3 as projected on the S-U plane) satisfies thecondition of d′=d. Therefore, the magnitude d′ is given by:d′=d=√(s′ ² +u′ ²)=√(s ² +u ²)  (10)

Further, the following equations are obtained:u′·tan 2ω′=t′  (11)u′·tan(θ2−φ1)=s′  (12)

From the equations (9) and (10), the following equation can be obtained:t′ ²=1−d ²  (13)From the equations (9), (12), and (13), the following equation can beobtained:u′ ²·{ tan²(θ2−φ1)+1}+(1−d ²)=1  (14)∴u′ ² =d ²/{ tan² (θ2−φ1)+1}  (15)From the equations (11) and (15), the following equation can beobtained:

$\begin{matrix}\begin{matrix}{{{2\;\omega^{\prime}}} = {{\tan^{- 1}\left( {t^{\prime}/u^{\prime}} \right)}}} \\{= {\tan^{- 1}\left\lbrack \left. \sqrt{}\left\lbrack {\left( {1 - d^{2}} \right){\left\{ {{\tan^{2}\left( {{\theta\; 2} - {\phi\; 1}} \right)} + 1} \right\}/d^{2}}} \right\rbrack \right. \right\rbrack}}\end{matrix} & (16)\end{matrix}$wherein d² in the equation (16) is defined in the equation (8).

Now, the second deflection from the light beam a3 to the light beam a4will be considered. See A—A section and B—B section (parallel to theincident plane) of FIG. 8. The following equation is obtained:tan γ=tan (θ2−φ1)·cos 2ω′  (17)

Since γ′ of C—C section satisfies the condition of γ′=γ, the followingequation is obtained:tan γ′=tan γ=tan (θ2−φ1)·cos 2ω′  (18)

When the C—C section is projected on the incident plane, the followingequation is obtained:tan (θ2+φ2)=tan γ′/cos (2ω2ω′)=cos 2ω′·tan (θ2−φ1)/cos (2ω+2ω′)  (19)As this equation (19) is expanded with regard to φ2, the followingequation is obtained:φ2=tan⁻¹{ cos 2ω′·tan(θ2−φ1)/cos(2ω+2ω′)}−θ2  (20)

From the equations (20), (16), and (8), the difference in exit angle φ2between the emergent light beam a4 when the emergent light beam a4 is onthe incident plane and a straight line as the emergent light beam a4being projected on the incident plane when the deflective reflectingfacet 11 is revolved by the maximum rotational angle ω can be obtainedby using known values.

FIG. 11 is a graph plotting the relation between θ1 and θ2 whichsatisfies Φ2=0 relative to cases of ω=7.5°, ω=10°, ω=12.5°, and ω=17.5°.The reason why the lower limit and the upper limit are 7.5° and 17.5° isthat the deflection angle of the deflected light beam a4 issubstantially ±4ω so that the minimum value and the maximum value arerealistically about ±30° and ±70°. As a result of FIG. 11, it is foundthat the change of curvature in scanning line trail at the optical axialdirectional position can be minimized by setting the relation between θ1and θ2 into a range represented by the following equation (21):0.33·θ1≦θ2≦0.37·θ1  (21)

In resent scanning optical devices of electrophotographic process,non-spherical molded plastic optical parts such as toric lenses aregenerally used for the scanning optical system 23 in order to reduce thenumber of lenses. However, it is difficult to achieve high precisionmanufacture of non-spherical optical surface as compared to themanufacture of spherical optical surface so that such a non-sphericaloptical surface has further small effective range in the sub-scanningdirection. Accordingly, it is needed to limit the curvature of scanningline trail within a smaller range of about 3 mm or less if such anoptical surface is used. To satisfy this condition, it is preferable toset the difference in exit angle Φ2 to be Φ2≦0.6°.

That is, for example, assuming that the deflective angle (entire angle)of the deflected light beam a4 is 65°, a focusing distance of 277 mm isrequired when the length of main scanning direction of the imagingsurface 24 is the length of the short side of A3 size paper plus thedistance for detecting synchronization signal of scanning, that is, 310mm. Assuming that the distance from the deflective reflecting facet 11to the part (scanning optical system 23) having non-spherical opticalsurface is up to about 277 mm which is equal to the focusing distanceand that Φ2=0.6° is satisfied, 277×tan(0.6°)=2.9 mm so that thecurvature in scanning line trail can be limited within 3 mm. Though thedistance from the deflective reflecting facet 11 to the optical surfaceof the scanning optical system 23 being 277 mm, the distance may not belimited to this value. The curvature in scanning line trail on theoptical surface can be limited to be about 3 mm by arranging the opticalpart having a surface, not a simple spherical surface, obtained byrelatively difficult process as the scanning optical system 23 to have adistance of 286 mm or less from the deflective reflecting facet 11 andΦ2≦0.6°.

FIG. 12 is a graph showing a boundary of a range satisfying Φ2=0.6°relative to the deflection angle (semiangle) of light beam of 50° orless. From FIG. 12, the following equation is obtained:0.33·θ1-1.27≦θ2≦0.35·θ1-1.50  (22)The realistic curvature in scanning line trail at the position of thescanning optical system 23 can be limited to be 3 mm or less by settingθ1 and θ2 to satisfy the relation of the equation (22).

When higher surface precision of optical surface in the scanning opticalsystem 23 is required in order to obtain higher imaging property, theoptical surface is disposed near the intersection P shown in FIG. 5 (anintersection of the emergent light beam a4 (solid line) when thedeflective reflecting facet 11 faces forthright and the emergent lightbeam a4 is on the incident plane and the emergent light beam a4 (brokenline) as the emergent light beam a4 being projected on the incidentplane when the deflective reflecting facet 11 is revolved by the maximumrotational angle ω) so that the curvature in scanning line trail on thelens surface near the intersection P is substantially zero, therebyachieving the scanning optical system 23 in which the required effectiverange in the sub-scanning direction is very small.

On the assumption that a rotary polygonal mirror (polygon mirror) 10 isused as the deflective reflecting facet 11, and that when the emergentlight beam a4 is on the incident plane, the optical length between thefirst deflective reflection point and the second deflective reflectionpoint is DL, the distance from the reflected point of the seconddeflection to the lens surface near the intersection P of the scanningoptical system 23 is S, and the distance from the deflective reflectingfacet 11 to the rotary axis 12 is R, the relation between the incidentangle θ1 and the exit angle θ2 making the curvature δ to be zero can becalculated.

FIG. 13( a) is an illustration for explaining a sag amount Δ1 at thefirst deflective reflection point and a sag amount Δ2 at the seconddeflective reflection point according to the revolution of thedeflective reflecting facet 11 and FIG. 13( b) is an enlarged viewshowing a portion near the first deflective reflection point, whereΔ′=Δ1+Δ2. In FIGS. 13( a), 13(b), solid lines indicate a position wherethe deflective reflecting facet 11 of the rotary polygonal mirror 10 isrevolved by the maximum rotational angle ω and broken lines indicate aposition where the deflective reflecting facet 11 faces forthright.

From FIG. 13( b), assuming that the rotary polygonal mirror 10 is usedas the deflective reflecting facet 11, Δ1 is calculated by the followingequation where the radius of an inscribed circle of the rotary polygonalmirror 10 is R:Δ1=R(1−cos ω)/cos ω  (23)

FIG. 14( a) is an illustration for explaining Δ′ which is the sag amountΔ1 at the first deflective reflection point plus the sag amount Δ2 atthe second deflective reflection point according to the revolution ofthe deflective reflecting facets and FIG. 14( b) is an enlarged view ofa portion near the second deflective reflection point. As D0 is taken asshown in FIG. 14( a), Δ′ is calculated by the following equation fromFIGS. 14( a), 14(b):

$\begin{matrix}{\Delta^{\prime} = {{{{D0} \cdot \tan}\;\omega} + {{{D0} \cdot \tan}\;{\omega \cdot \tan}\; 2\;{\omega \cdot \tan}\;\omega} + {{{D0} \cdot \tan}\;{\omega \cdot \left( {\tan\; 2\;{\omega \cdot \tan}\;\omega} \right)^{2}}} + \ldots}} & (24)\end{matrix}$Since this equation is an endless addition in geometrical progressionwith the first term of D0·tan ω and the geometrical ratio of tan 2ω·tanω, the following equation is obtained:Δ′=D0·tan ω/(1−tan 2ω·tan ω)  (25)That is,

$\begin{matrix}{{\Delta\; 2} = {{{\Delta\;}^{\prime} - {\Delta\; 1}} = {{{{{D0} \cdot \tan}\;{\omega/\left( {1 - {\tan\; 2\;{\omega \cdot \tan}\mspace{11mu}\omega}} \right)}} - {\Delta\; 1}} \approx {\left\lbrack {{\left\{ {{{{DL}\left( {{\cos\;\theta\; 1} + {\cos\;\theta\; 2}} \right)}/2} - {{2 \cdot \Delta}\; 1}} \right\} \cdot \mspace{25mu}\tan}\; 2\;\omega \times \tan\;{\omega/\left( {1 - {\tan\; 2\;{\omega \cdot \tan}\;\omega}} \right)}} \right\rbrack - {\Delta\; 1}}}}} & (26)\end{matrix}$The curvature δ in the optical surface at this time is represented bythe following equation based on FIGS. 15( a), 15(b). FIGS. 15( a), 15(b)are illustrations showing shifting amounts of reflected light beams a1,a4 according to the sag amount Δ1 at the first deflective reflectionpoint and the sag amount Δ2 at the second deflective reflection point.In the equation, φ1 and φ2 are both zero.

$\begin{matrix}{\delta = {{{S \cdot \tan}\;\phi\; 2} - {{{DL} \cdot \tan}\;{\phi 1}} + {\Delta\;{1 \cdot {{\sin\left( {{{2 \cdot \theta}\; 1} + {\phi\; 1}} \right)}/\cos}}\;\theta\; 1} + {\Delta\;{2 \cdot {{\sin\left( {{{2 \cdot \theta}\; 2} + {\phi\; 2} - {\phi\; 1}} \right)}/{\cos\left( {{\theta\; 2} - {\phi\; 1}} \right)}}}}}} & (27)\end{matrix}$

The first paragraph of this equation is based on the difference betweenthe exit angles before and after the second reflection and the secondparagraph is based on the difference between the exit angles before andafter the first reflection. Though these values are not so large, thedifferences lead to a curvature in scanning line trail between the firstθdeflective reflection point and the second deflective θreflection pointas mentioned above. The third paragraph is based on the sag amount Δ1 atthe first deflective reflection point and the fourth paragraph is basedon the sag amount Δ2 at the second deflective reflection point. In theequation, φ1 is a value obtained by the equation (2) and φ2 is a valueobtained by the equation (20).

From the equations (27), (23), and (26), when it is assumed that theoptical length between the first deflective reflection and the seconddeflective reflection is DL=20 mm, the distance from the seconddeflective reflection point to the optical surface is S=100 mm, thedistance from the deflective reflecting facet 11 to the rotational axis12 thereof is R=20 mm, and the maximum deflection angle of thedeflective reflecting facet 11 is ω=15°, the relation between θ1 and θ1making the curvature δ to be substantially zero can be obtained and isshown in FIG. 16. The curvature δ in scanning line trail on the opticalsurface becomes substantially zero if the relation represented by thefollowing equation is satisfied:θ2=0.39·θ1  (28)

Though the parameters are set to be values for an image formingapparatus such as an actual laser beam printer in the aforementionedembodiment, the present invention is not limited to the equation (28)and the aforementioned values set for the parameters.

Though the aforementioned study was made about the light deflectiveoptical system in which a light beam a0 is incident on the deflectivereflecting facet 11 through the space 15 between the two stationaryplane mirrors 13 and 14 and a light beam a4 exits through the space 15between the stationary plane mirrors 13 and 14, the study is alsoadopted to a light deflective optical system in which a light beam a0 isincident from above or below two stationary plane mirrors 13 and 14 anda light beam a4 exits through a space 15 between the stationary planemirrors 13 and 14 such that the stationary plane mirror 14 as one of thestationary plane mirrors 13 and 14 is disposed to be sandwiched betweenthe incident light beam a1 and the emergent light beam a4 or a lightdeflective optical system having a light path contrary to the above.

The arrangement shown in FIG. 4 has an advantage that there is nolimitation in size of the stationary plane mirrors 13, 14 so as to allowthe use of inexpensive mirrors. The arrangement shown in FIG. 17 has anadvantage that the first and second deflective reflection points can bebrought to close to each other so as to reduce the dimension in therotational axial direction of the deflective reflecting facet 11.

An optical scanning device according to a second embodiment of thepresent invention will be described with reference to FIG. 18.

FIG. 18 is a perspective view showing the entire structure of theoptical scanning device according to another embodiment of the presentinvention and FIG. 19 is a perspective view showing a light deflectiveoptical system as main parts of the optical scanning device.

In the optical scanning device according to the first embodiment of thepresent invention as mentioned above, the lens with numeral 22 in FIG. 1is a collimator lens so that the light beam converted by the lens 22 isa parallel beam which is parallel to both the direction perpendicular toand the direction parallel to the rotational axis 12. In the opticalscanning device according to the second embodiment, however, a lens withnumeral 22′ in FIG. 18 is an anamorphic lens. This is a point differentfrom the first embodiment.

According to this structure, an optical deflector is composed of apolygon mirror 10 taking a form of a polygonal column and having aplurality (six in the illustrated example) of deflective reflectingfacets 11 on the peripheral sides of the polygonal column. The polygonmirror 10 rotates about its rotational axis 12 so that the deflectivereflecting facets 11 revolve about the rotational axis 12. Twostationary plane mirrors 13, 14 are disposed to face a deflectivereflecting facet 11 related to optical deflection so that these planemirrors 13, 14 have an angle relative to each other and have a space 15therebetween.

A light beam a0 from a light source 21 is converted by the anamorphiclens 22 into a light beam, which is parallel to the directionperpendicular to the rotational axis 12 and converges at a portion nearthe second reflection point on the deflective reflecting facet 11relative to the direction parallel to the rotational axis 12, and isincident on the deflective reflecting facet 11 diagonally from below, incase of a drawing taken along the rotational axis 12, through the space15 between the stationary plane mirrors 13 and 14. The light beam a0becomes a light beam a1 after the first reflection by the deflectivereflecting facet 11. The light beam a1 proceeds diagonally upward to beincident on one stationary plane mirror 13. The light beam a1 becomes alight beam a2 after reflection at the stationary plane mirror 13. Thelight beam a2 proceeds downward to be incident on the other stationaryplane mirror 14. The light beam a2 becomes a light beam a3 afterreflection at the stationary plane mirror 14. The light beam a3 isincident on the deflective reflecting facet 11 again. The light beam a3becomes a light beam a4 after the second reflection by the deflectivereflecting facet 11. The light beam a4 proceeds diagonally upwardthrough the space 15 between the stationary plane mirrors 13 and 14 andis converted into a focused light beam via a scanning optical system 23and is focused to be incident on an imaging surface 24. Since thedeflective reflecting facets 11 revolve about the rotational axis 12,the focused light beam moves at a rotational speed about four times asfaster as the rotational speed of the deflective reflecting facets 11 towrite a scan line b on the imaging surface 24. Adjacent deflectivereflecting facets 11 successively come in and go away the position ofincidence of the incident light beam a0 because of the rotation of thepolygon mirror 10. As a result, according to the rotation of the polygonmirror 10, the scan lines b are successively written from one end to theother end on the imaging surface 24 at the same level. The scanning inthis direction is called the main scanning. Sub scanning is conducted bymoving a scanned substrate on the imaging surface 24 in a directionperpendicular to that of the main scanning at a constant rate. The mainscanning and the sub scanning cooperate to achieve raster scanning inwhich scan lines b are aligned with a constant pitch on the scannedsubstrate.

The two stationary plane mirrors 13, 14 are disposed perpendicularly tothe incident plane on the condition that the incident plane is definedas a plane being parallel to the rotational axis 12 and including thecentral ray of the incident light beam a0.

Similarly to the optical scanning device of the first embodiment, theabove description made with reference to FIG. 1 through FIG. 10 can beadopted to the optical scanning device of the second embodiment and theequations (1) through (20) are also adopted.

FIG. 20 is a graph, as an example, plotting differences in exit angle Φ2relative to the rotational angle ω of the deflective reflecting facet 11caused by changing the ratio between θ1 and θ2 according to the equation(20). Except some special cases of ratio between θ1 and θ2, the absolutevalue of the difference in exit angle Φ2 increases as the deflectivereflecting facet 11 is revolved from the facing forthright position(ω=0°). That is, a curvature similar to that in FIG. 3 is produced alsoin scanning line trail of light beam a4 after the second reflection atthe deflective reflecting facet 11.

As described above, the amount of curvature in scanning line trail ofthe light beam a4 after the second reflection at the deflectivereflecting facet 11 can be minimized by setting the ratio between θ1 andθ2 into a range represented by the following equation (21):0.33·θ1≦θ2≦0.37·θ1  (21)

FIGS. 21( a), 21(b) are illustrations showing the light beam a4 from thedeflective reflecting facet 11 to an imaging surface 24 through thescanning optical system 23, the light beams a4 being illustrated in astate projected on an incident plane. In the incident plane, FIG. 21( a)is a case when the scanning optical system 23 is disposed such that thesecond deflection point P2 on the deflective reflecting facet 11 issubstantially conjugated to the imaging surface 24 and FIG. 21( b) is acase when the scanning optical system 23 is disposed, for example infront of the focal point, such that the second deflection point P2 isnot conjugated to the imaging surface 24. In FIGS. 21( a), 21(b), theemergent light beam a4 indicated by solid lines is the emergent lightbeam a4 when the deflective reflecting facet 11 faces forthright and theemergent light beam a4 indicated by broken lines is the emergent lightbeam a4 when the deflective reflecting facet 11 is revolved by arotational angle ω from the facing forthright position.

As described above, in the deflective optical system in which anincident light beam is reflected by two stationary plane mirrorssequentially so that the light beam is incident twice on the samedeflective reflecting facet to deflect the light beam, there is nonecessary to provide an optical system for correcting the tilt error.However, unless the light beam a3 is incident on the deflectivereflection facet 11 at the second deflection point P2 at an angle notperpendicular to the sub scanning direction (θ2≠0°), a difference inexit angle Φ2 as mentioned above must be generated so as to causecurvature in scanning line trail. As shown in FIG. 21( b), the scanningoptical system 23 is disposed such that the second deflection point P2on the deflective reflecting facet 11 is not conjugated to the imagingsurface 24. The position in the sub scanning direction of the emergentlight beam a4 converging on the imaging surface 24 varies according tothe rotational angle ω so as to cause a curvature in scanning line.

To solve this problem, in the second embodiment of the present inventionas shown in FIG. 21( a), the scanning optical system 23 is disposed insuch a manner that the second deflection point P2 on the deflectivereflecting facet 11 is substantially conjugated to the imaging surface24 and the anamorphic lens 22 (FIG. 1) is disposed in optical path insuch a manner that a light beam from the light source 21 converges at aportion near the second deflection point P2 in the sub scanningdirection parallel to the rotational axis 12. The lens 22 may becomposed of an optical system as a combination of a collimator lenswhich is rotational symmetric and a cylindrical lens having conversingeffect only in the sub scanning direction can be used instead of theanamorphic lens 22. As the scanning optical system 23 is disposed insuch a manner that the second deflection point P2 on the deflectivereflecting facet 11 is substantially conjugated to the imaging surface24, the position in the sub scanning direction of the emergent lightbeam a4 converging on the imaging surface 24 does not move even withvariation of rotational angle ω. Therefore, even when the scanning linetrail is curved due to the difference in exit angle φ2, no curvature inscanning line is caused so as to create a substantially straightscanning line b on the imaging surface 24.

FIG. 22( a) is an illustration similar to FIG. 5, but showing a casewhere the deflective reflecting facet 11 has a surface tilt error by anangle ε. Solid lines indicate a case where there is no surface tilterror and broken lines indicate a case where there is a surface tilterror by an angle ε. Even when the deflective reflecting facet 11 has asurface tilt error, the emergent light beam a4 projected on the incidentplane moves just in parallel while the second deflection point on thedeflective reflection facet 11 moves from P2 to P2′. With reference toFIG. 22( b) as an auxiliary drawing, the moving amount Y is given by:Y=Ld·tan (2ε)/cos (2ε)  (28)where Ld is the transfer distance from the first deflection point to thesecond deflection point when the emergent light beam a4 is on theincident plane. Assuming that the second deflection point on thedeflective reflection facet 11 moves the above amount, the scanning lineb moves a distance Y′ by the focusing at the second reflection point inthe sub scanning direction of the scanning optical system 23. Thedistance Y′ is given by:Y=β·Ld·tan (2ε)/cos (2ε)  (29)where β is the transverse magnification in the sub scanning direction ofthe scanning optical system 23.

The distance Y′ is a displacement of the scanning line on the imagingsurface 24 due to the surface tilt error of the deflective reflectingfacet 11 at the second deflection point. Therefore, the displacementmust be about ¼ or less of the scanning line pitch LP in case of amonochrome electrophotographic apparatus in which half tone in densitiesis not important and about ⅛ or less of the scanning line pitch LP incase of color electrophotographic apparatus in which half tone indensities is important. Taking these into consideration, it is requiredto satisfy the following equation:

for the monochrome electrophotographic apparatus,|β·Ld·tan (2ε)/cos (2ε)|≦0.25·LP  (30)

for the color electrophotographic apparatus,|β·Ld·tan (2ε)/cos (2ε)|≦0.125·LP  (31)

By the way, as shown in FIG. 24, when the deflective reflecting facet 11is not a right plane as shown by a solid line and is twisted or curvedas shown by a broken line, the emergent light beam a4 as projected onthe incident plane moves not in parallel as shown in FIG. 22( a) but atan angle to the emergent light beam a4 when the deflective reflectingfacet is a right plane. The difference in exit angle causes curvature inscanning line trail. Further, if the degree of twist or the degree ofcurvature varies every deflective reflecting facet, the angle to theemergent light beam a4 when the deflective reflecting facet 11 is aright plane also varies every deflective reflecting facet 11. Thevariation of the exit angle causes variation in the position of thescanning line trail in the sub scanning direction.

Like the second embodiment of the present invention, however, bydisposing the scanning optical system 23 in such a manner that thesecond deflection point P2 on the deflective reflecting facet 11 issubstantially conjugated to the imaging surface 24 and by disposing theanamorphic lens 22 (FIG. 1) in optical path in such a manner that alight beam from the light source 21 converges at a portion near thesecond deflection point P2 in the sub scanning direction parallel to therotational axis 12, the scanning line b on the imaging surface 24 can besubstantially straight even when the deflective reflecting facet hastwist and/or curvature. There is also such an effect that, even when thedegree of twist or the degree of curvature varies every deflectivereflecting facet, the scanning line distortion of the scanning line bformed by emergent light beam a4 converged on the imaging surface 24 canbe corrected.

FIGS. 25( a), 25(b) are illustrations in which incident light beam a0and the emergent light beam a4 are projected on the incident plane nearthe polygon mirror 10, corresponding to FIGS. 21( a), 21(b),respectively. In case that the second deflection point P2 on thedeflective reflecting facet 11 is set to be substantially conjugated tothe imaging surface 24 as shown in FIG. 25( a), the light beams a0 anda4 passing through the space 15 between the stationary plane mirrors 13and 14 have reduced beam diameter in the sub scanning direction becauseof the convergence near the space 15. On the other hand, in case thatthe second deflection point P2 on the deflective reflecting facet 11 isset not to be conjugated to the imaging surface 24, the light beams a0and a4 passing through the space 15 between the stationary plane mirrors13 and 14 have not reduced beam diameter (thick relative to the beamdiameter of the above case) as shown in FIG. 25( b). Therefore, in thesecond embodiment of the present invention shown in FIG. 25( a), thedistance between the first deflection point P1 and the second deflectionpoint P2 on the deflective reflecting facet 11 can be reduced bydisposing the stationary plane mirrors 13 and 14 at positions nearer tothe polygon mirror 10. In addition, since the converged point in the subscanning direction is positioned near the deflective reflecting facet11, the beam diameter in the sub scanning direction is reduced.Therefore, the second embodiment of the present invention shown in FIG.25( a) allows reduction in the thickness D of the polygon mirror 10 inthe direction of the rotational axis 12, thereby providing advantages ofreducing the size and the cost of the optical scanning device.

Further, since the light deflective optical system is structured suchthat the light beam a0 is incident on the deflective reflecting facet 11from the illumination optical system through the space 15 between thetwo stationary plane mirrors 13 and 14 and the light beam a4 exitsthrough the space 15, there is no limitation on the size opposite to thespace 15 between the stationary plane mirrors 13 and 14. Therefore, thetransfer distance between the first deflective point P1 and the seconddeflective point P2 can be shortened without using mirrors each of whichhas a highly accurate and very small reflecting surface requiringdifficult process and which is thus expensive.

Though the above description was made for a case that the polygon mirror(rotary polygonal mirror) is employed as the deflective reflecting facet11, the aforementioned effects can be exhibited even in a case that aswiveling galvanometer mirror is employed.

Though the principle and embodiments of the optical scanning deviceaccording to the present invention have been described in the above, thepresent invention is not limited thereto and various modifications maybe made.

FIG. 26 is an explanatory view showing an example of the opticalscanning device 50 as described above. In FIG. 26, a light beam a0outputted from a light source (not shown) is reflected by a reflectivemirror 56 so that the light beam a0 is incident on a rotary polygonalmirror 10 diagonally upwardly not in a direction perpendicular to arotational axis 12 of the rotary polygonal mirror 10.

The incident light beam a0 into the rotary polygonal mirror 10 isreflected at a deflective reflecting facet 11 to proceed diagonallyupwardly as a deflected light beam a4 in the drawing. The deflectedlight beam a4 is incident on a reflective mirror 57 via the scanningoptical system (image formation lens) 23.

The incoming beam into the reflective mirror 57 is reflected to proceeddiagonally downwardly to the right in the drawing toward a secondreflective mirror 58 positioned on the deflective reflecting facet sideand is reflected at the second reflective mirror 58 to proceed upwardlyin the drawing. The light beam proceeding upwardly is incident on animaging surface 24 such as a photoreceptor drum which rotates in thedirection of arrow A. The light beam into the imaging surface 24 writesa scanning line in a direction perpendicular to the direction of paperof the drawing.

Numeral 51 designates a driving motor for the rotary polygonal mirror 10and numeral 60 designates a body frame supporting the rotary polygonalmirror 10, the driving motor 51, the scanning optical system (imageformation lens) 23, and the reflective mirrors 56 through 58. Numeral 71designates a cover glass mounted portion to which a cover glass forpermitting the transmission of scanning beam is mounted.

FIG. 45 is a schematic perspective view showing an example of the coverglass mounted portion 71. In FIG. 45, the cover glass 72 is mounted to amounting member 73 via a frame 74. The reason why the cover glass 72 ispositioned in a recess relative to the mounting member 73 is that thecover glass 72 is prevented from being damaged by contact with othermembers.

In addition, another reason is that the possibility that an operatorcontaminates the surface of the cover glass by touching with hands isreduced. The surface of the cover glass is cleaned by a cleaning memberexclusively for the cover glass in order to prevent the transmission oflight being blocked due to adhesion of dust and dirt. In the opticalscanning system having the cover glass for permitting the transmissionof light from the optical system to the imaging surface, the cover glasscleaning mechanism is provided between the optical system for lightscanning and the imaging surface.

As mentioned above, the cover glass 72 is disposed in the recess formedby the frame 74 at a level lower than the surface of the mounting member73. Therefore, after dust and dirt adhering to the surface of the coverglass 72 are wiped with the cleaning member, the dust and dirt stillremain in the frame 74. That is, there is a problem that it isimpossible to effectively clean the cover glass 72.

With reference to some drawings, a cover glass cleaning mechanism for anoptical scanning device according to the present invention will bedescribed. FIG. 27 is a schematic perspective view showing a cover glasscleaning mechanism according to an embodiment of the present invention.In FIG. 27, the cover glass cleaning mechanism 31 has a structure that acover glass 32 is disposed to project on a mounting member 33 (see FIGS.30( a), 30(b)).

A guide member 36 is mounted on the mounting member 33 and has astructure that a cleaning lever 35 can reciprocate within the guidemember 36. The cleaning lever 35 is provided at its distal end 35 a witha cleaning member 37 and a cushion member 38 as shown in FIGS. 30( a),30(b). Numeral 35 b designates a proximal end of the cleaning lever 35.

Numeral 34 partially indicates the side wall of the body frame. The sidewall has an opening 34 a through which the cleaning lever 35 can move.Numeral 41 designates a convex portion for retaining the cleaning lever35 and numeral 43 designates a stopper for preventing the cleaning lever35 from coming off. The details of the convex portion 41 and the stopper43 will be described later.

FIG. 28 is a schematic perspective view showing a state that the distalend 35 a of the cleaning lever 35 is withdrawn to the position of thebody frame 34. FIG. 29 is a schematic perspective view showing a statethat the proximal end 35 b is brought to the position of the body frame34 by pushing the cleaning lever 35 along the guide member 36.

FIGS. 30( a), 30(b) are schematic sectional front views partiallyshowing the cover glass cleaning mechanism. FIG. 30( b) shows a statethat the cleaning member 37 is on the cover glass 32. FIG. 30( a) showsa state that the cleaning lever 35 is out of the cover glass 32 as aresult of that the cleaning lever 35 proceeds in the direction of arrowB.

The cleaning member 37 has flanges 37 a, 37 b formed on the both sides.The flanges 37 a, 37 b are engaged with retaining members 39 a, 39 battached to the distal end 35 a of the cleaning lever 35, therebyretaining the cleaning member 37. The cushion member 38 is accommodatedbetween an inside concavity 35 c of the cleaning lever 35 and thecleaning member 37.

As the cleaning lever 35 proceeds from the position shown in FIG. 30( b)in the direction of arrow B along the guide member (not shown), thecleaning member 37 removes dust and dirt adhering to the surface of thecover glass 32 with some pressure from the cushion member 38 so as towipe the dust and dirt. As the cleaning member 37, artificial lathersuch as back skin having a raised surface can be used. Since thecleaning member 37 is pressed against the surface of the cover glass 32by an elastic member such as the cushion member 38, dust and dirtadhering to the surface of the cover glass 32 can be effectivelyremoved.

In the state that the cleaning lever 35 reaches the position shown inFIG. 30( a), that is, the position where the cleaning lever 35 is out ofthe cover glass, the cleaning member 37 is positioned at a level lowerthan the level of the surface of the cover glass 32 because of theelasticity of the cushion member 38. Consequently, the dist and dirtwiped off from the cover glass 32 and attached to the cleaning member 37fall out of the cover glass 32.

In the present invention, the stroke distance of the cleaning lever 35is set to be such a length that the cleaning member 37 accommodated inthe concavity 35 c formed in the distal end 35 a reaches a position outof the end of the cover glass 32. According to this structure, afterdust and dirt adhering to the surface of the cover glass are wiped bythe cleaning member 37, the dust and dirt do not remain on the coverglass.

When the cleaning lever 35 is returned to the initial position, thecleaning member 37 is put on the cover glass 32 via the end edge 32 a ofthe cover glass 32. Accordingly, even if dust and dirt remain on thecleaning member 37, the dust and dirt are efficiently scraped by the endedge 32 a of the cover glass 32, whereby the surface of the cleaningmember 37 can be maintained clean even after repeated cleaningoperation. That is, according to the present invention, the strokedistance of the cleaning lever 35 is set to allow the movement of thecleaning member at the position out of one of the ends of the coverglass.

FIG. 31 is a schematic perspective view showing a state that the guidemember 36 for the cleaning lever 35 is mounted on the mounting member33. The guide member 36 has a function of keeping the accuracy ofposition between the cover glass 32 and the cleaning mechanism high byretaining the cleaning lever 35 not to shift vertically or laterallyfrom the normal position on the cover glass. Since the guide member 36is arranged above the cover glass 32, the guide member 36 also has afunction of preventing the cover glass from being contaminated by handsof an operator or adhesion of foreign matter.

FIG. 32 is a schematic cross sectional view showing an example of theguide member 36. As shown in FIG. 32, the guide member 36 is formed inan arch shape so as to cover the cover glass 32 and has legs 36 a, 36 bwhich are fixed to the mounting member 33. The guide member 36 extendsover the entire length of the cover glass 32. The cleaning lever 35reciprocates parallel to the cover glass 32 along the inner space formedin the arch shape of the guide member 36. Therefore, the cleaning member37 is pressed against the cover glass 32 with constant pressure, therebyproperly cleaning the cover glass 32 uniformly without remainingnon-cleaned portions.

Accordingly, there is an advantage that the cleaning member 37 does notshift vertically or laterally from the normal position on the coverglass 32 so that the accuracy of position between the cleaning member 37and the cover glass 32 can be maintained high. Since the cleaning lever35 is retained by the guide member 36 extending over the entire lengthof the cover glass 32, the cleaning member 37 can be prevented fromcoming off the cleaning lever 35 and thus prevented from beingcontaminated by touching portions other than the cover glass 32.

FIG. 33 is a schematic cross sectional view showing an example of theguide member 36 according to another embodiment. As shown in FIG. 33, aguide member 36X has projections 36 c, 36 d above and on the both sidesof the cover glass 32. A cleaning lever 35 has grooves 35 p, 35 q formedin the both side surfaces thereof. The grooves 35 p, 35 q are fittedwith the projections 36 c, 36 d of the guide member 36X, therebylimiting the position of the cleaning lever 35 from unnecessarilyshifting. This example also can maintain the accuracy of positionbetween the cleaning member 37 and the cover glass 32 high.

FIG. 34 is a schematic perspective view showing a cleaning lever 35according to an embodiment of the present invention. In FIG. 34, thecleaning lever 35 is provided at the center with a slot which is long inthe longitudinal direction. The slot functions as a scanning lightwindow 40. The scanning light window 40 permits the transmission oflight beams to be incident on a photoreceptor drum so as to allow imageformation even when the cleaning lever 35 is fully retracted inside asshown in FIG. 29.

Since the optical scanning device is always available even in a statethat the cleaning lever 35 is attached, there is no need to detach andstore the cleaning lever 35, thereby preventing the cleaning lever 35from being missing and further preventing a storing place from beingcontaminated by the cleaning lever 35.

FIG. 35 is a schematic perspective view partially showing one of theends in the longitudinal direction of the guide member 36. In FIG. 35,the end in the longitudinal direction of the guide member 36 is providedwith cutting grooves 36 p, 36 q which are parallel to each other. Theend of a portion between the cutting grooves 36 p and 36 q is deformedby bending process so as to form a convex portion 41. The end of theguide member 36 is formed with a bent portion 42 having an L-likesection.

FIG. 36 is a schematic perspective view partially showing one of theends in the longitudinal direction of the cleaning lever 35. In FIG. 35,the cleaning lever 35 is provided with a concave portion 35 x at the oneend 35 a in the longitudinal direction of the cleaning lever 35. Theconcave portion 35 x is formed at such a position that the concaveportion 35 x confronts the convex portion 41 formed at the end of theguide member 36 mentioned above when the cleaning lever 35 is fullyretracted inside and thus functions as a movement restriction means.

FIG. 37 is a sectional front view partially showing the relation betweenthe guide member 36 and the cleaning lever 35 when the cleaning lever 35is fully retracted. In this example, the cleaning member 37 is shown ina state positioned out of the end of the cover glass 32. As shown inFIG. 37, the convex portion 41 formed in the guide member 36 is engagedwith the concave portion 35 x formed in the cleaning lever 35 so as tostop the cleaning lever 35 from unnecessarily moving when the cleaninglever 35 is retracted inside.

This structure can prevent such an event that dust and dirt attached tothe cleaning member 37 fly to the surface of the cover glass 32 due tovibration of the cleaning lever 35. In addition, the engaging meanscomposed of the aforementioned concave and convex portions alsofunctions as a click mechanism so that an operator can sensuously checkwhen the cleaning lever 35 reaches the traveling terminal positionduring pushing the cleaning lever 35.

The guide member 36 is provided at its end with a bent portion 42 havingan L-like section which functions as a stopping member which stops thefurther movement of the cleaning lever 35 at the terminal end so as toconduct the movement restriction, thereby preventing the excessivemovement of the cleaning lever 35. Therefore, the economical movement ofthe cleaning lever 35 can be achieved.

The end 37 b of the cleaning member 37 and the retaining member 39 b areboth provided with saw teeth which mesh with each other, therebysecurely retaining the cleaning member 37.

FIG. 38 is a sectional front view similar to FIG. 37, but showinganother embodiment. Only different points from the embodiment of FIG. 37will be described. In the embodiment of FIG. 38, a mounting member 33 isprovided with a convex portion 46. The cleaning lever 35 is providedwith a concave portion 35 y, as a movement restriction means, at aposition corresponding to the convex portion 46 such that the concaveportion 35 y and the convex portion 46 can be engaged with each other.

In the present invention, it is possible to form the movementrestriction means in the cleaning lever and to form the engaging meansin the guide member, but not shown. It is also possible to form themovement restriction means in the cleaning lever and to form theengagement means in the mounting member for the cover glass.

The mounting member 33 is provided at the end with a bent portion 42 afunctioning as a stopping member for limiting the terminal end of thecleaning lever 35. In the embodiment of FIG. 38, the mounting member 33is structured to function as a cover for a laser scanning unit (LSU),thereby reducing the number of parts.

FIG. 39 is a schematic perspective view showing an embodiment of thepresent invention and FIG. 40 is a schematic cross sectional front viewof the embodiment of FIG. 39. In FIG. 39 and FIG. 40, a cleaning lever35 is provided with a stopper 43 on the surface thereof. The stopper 43comes in contact with the wall of the body frame 34 when the cleaninglever 35 is withdrawn, thereby preventing the cleaning lever 35 fromcoming off the guide member 36.

That is, as shown in FIG. 40, the cleaning member 37 moves to a positionout of the proximal end (near the body frame 34) of the cover glass 32by withdrawing the cleaning lever 35. In this state, the distal end ofthe cleaning lever 35 is positioned near the proximal end of the guidemember 36 and, if the cleaning lever 35 is further withdrawn, it mustcome off the guide member 36.

In this case, it is required to align the cleaning lever 35 to the guidemember 36 for the operation of pushing the cleaning lever 35 again, thusmaking the operation complex. However, the stopper 43 is formed in thecleaning lever 35. The movement of the cleaning lever 35 is restrictedat the position of the body frame 34 by the stopper 43 so that thecleaning lever 35 never comes off the guide member 36. Therefore, theoperation of pushing the cleaning lever 35 again is not troublesome,thus increasing the operability.

FIG. 41 is a perspective view showing details of the stopper 43 and FIG.42 is a schematic sectional front view showing the operation of thestopper 43. As shown in FIG. 41, a cutting groove 43 x is formed in thesurface of the cleaning lever 35 so as to form an elastic deformableportion 43 b. The stopper 43 has a hook portion 43 a formed on the endthereof.

The stopper 43 is provided with an elastic deformable portion 43 b.Therefore, as shown in FIG. 42, the elastic deformable portion 43 can bepivotally moved in the direction of arrow C by pressing the elasticdeformable portion 43, thereby allowing the hook portion 43 a to comeoff the body frame 34. Therefore, the cleaning lever 35 can come off theend of the guide member 36 so that the cleaning lever 35 can be removedfrom the body of the optical scanning device.

Since the cleaning lever 35 is removable from the optical scanningdevice in this manner, the convenience in maintenance such as cleaningof the cleaning member 37 and the replacement of the cleaning member 37with deterioration is improved.

FIG. 43 is an exploded perspective view of the cleaning lever 35 andFIG. 44 is a schematic perspective view showing the cleaning lever 35taken from below. As shown in FIG. 43, the cleaning member 37 and thecushion member 38 are attached to a holding plate 47. The both side endsof the holding plate 47 are bent to form legs 44 a, 44 b.

The cleaning lever 35 is provided with insertion holes 45 a, 45 b formedat positions corresponding to the legs 44 a, 44 b. The legs 44 a, 44 bof the holding plate 477 are inserted into the insertion holes 45 a, 45b, respectively so that the cleaning member 37 projects from the bottomof the cleaning lever 35. Since the cleaning member 37 is detachablyattached to the cleaning lever 35 as mentioned above, the inspection andthe replacement of the cleaning member 37 can be easily conducted.

The above-described embodiments are only examples of embodiments. Thepresent invention is not limited to the above-described embodiments andallows various modifications.

INDUSTRIAL APPLICABILITY

Apparent from the above description, according to the first throughthird optical scanning devices of the present invention, an opticalscanning device employing a light deflective optical system of twiceincidence type comprising a deflective reflecting facet and twostationary plane mirrors is characterized in that the central ray of anemergent light beam when the emergent light beam after the secondreflection by the deflective reflecting facet is on the incident planeand a straight line as the central ray of an emergent light beam beingprojected on the incident plane when the deflective reflecting facet isrevolved by the maximum rotational angle are set substantially parallelto each other, the equation (22) is satisfied, or the central ray of anemergent light beam when the emergent light beam after the secondreflection by the deflective reflecting facet is on the incident planeand a straight line as the central ray of an emergent light beam beingprojected on the incident plane when the deflective reflecting facet isrevolved by the maximum rotational angle intersect with each other at aposition near at least one of optical surfaces disposed in the opticalaxial direction of a scanning optical system. Therefore, curvature inscanning line trail on the optical surface(s) in the scanning opticalsystem is reduced or becomes substantially zero, thereby increasing thedegree of freedom of position arrangement of the scanning optical systemand allowing the reduction in dimension in the sub scanning direction ofa scanning optical system, thus achieving an inexpensive scanningoptical device which is small and has high accuracy.

According to the fourth optical scanning device of the presentinvention, an optical scanning device employing a light deflectiveoptical system of twice incidence type comprising a deflectivereflecting facet and two stationary plane mirrors is characterized inthat an emergent light beam when the emergent light beam after thesecond reflection by said deflective reflecting facet is on the incidentplane has such a relation that the second reflection point and theimaging surface are substantially conjugated to each other on theincident plane, whereby the position in the sub scanning direction ofthe emergent light beam converging on the imaging surface does not moveeven with variation of rotational angle of the deflective reflectingfacet. Therefore, even when the scanning line trail is curved due to thedifference in exit angle, no curvature in scanning line is caused so asto create a substantially straight scanning line on the imaging surface.In addition, even when there is difference in exit angle or variation inexit angle based on the twist or curvature of deflective reflectingfacet, a substantially straight scanning line is created in the imagingsurface, thereby preventing occurrence of variation in scanning line.The variation in scanning line due to surface tilt error of thedeflective reflecting facet can be minimized. With employing inexpensiveand highly accurate stationary plane mirrors, the dimension in therotational axis direction of the deflective reflecting facet is reduced,thereby achieving the reduction in size and in cost of the opticalscanning device.

According to the cover glass cleaning mechanism for an optical scanningdevice of the present invention, the cover glass cleaning mechanism foran optical scanning device is structured such that dust and dirtadhering to the surface of the cover glass can be securely removed.

1. An optical scanning device comprising a light deflective opticalsystem in which two stationary plane mirrors are disposed to face adeflective reflecting facet which s parallel to a rotational axis andcan be rotated or swivelled about the rotational axis such that a lightbeam being incident on and reflected from said deflective reflectingfacet is reflected by said two stationary plane mirrors sequentially andis incident on and reflected by the deflective reflecting facet again,wherein assuming that a plane being parallel to said rotational axis andincluding the light beam to be first incident on said deflectivereflecting facet is an incident plane, said two stationary plane mirrorsare disposed perpendicular to said incident plane, and the central rayof an emergent light beam when the emergent light beam after the secondreflection by said deflective reflecting facet is on said incident planeand a straight line as the central ray of an emergent light beam beingprojected on the incident plane when the deflective reflecting facet isrevolved by the maximum rotational angle are set substantially parallelto each other, wherein said optical scanning device is structured suchthat the light beam to be first incident on said deflective reflectingfacet passes through a space between said two stationary plane mirrorsand the deflected light beam after the second reflection by saiddeflective reflecting facet passes through the space between said twostationary plane mirrors.
 2. An optical scanning device comprising alight deflective optical system in which two stationary plane mirrorsare disposed to face a deflective reflecting facet which is parallel toa rotational axis and can be rotated or swivelled about the rotationalax is such that a light beam being incident on and reflected from saiddeflective reflecting facet is reflected by said two stationary planemirrors sequentially and is incident on and reflected by the deflectivereflecting facet again, wherein assuming that a plane being parallel tosaid rotational axis and including the light beam to be first incidenton said deflective reflecting facet is an incident plane, said twostationary plane mirrors are disposed perpendicular to said incidentplane, and the central ray of an emergent light beam when the emergentlight beam after the second reflection by said defective reflectingfacet is on said incident plane and a straight line as the central rayof an emergent light beam being projected on the incident plane when thedeflective reflecting facet is revolved by the maximum rotational angleare set substantially parallel to each other, wherein said opticalscanning device is structured to satisfy the following relation:0.33·θ1≦θ2≦0.37·θ1  (21) where θ1 is the incident angle of the centralray of the light beam to be first incident on said deflective reflectingfacet at a position where said deflective reflecting facet isperpendicular to said incident plane and θ2 is the exit angle of thecentral ray of the emergent light beam after the second reflection bysaid deflective reflecting facet.
 3. An image forming apparatusemploying an optical scanning device as claimed in claim 2 as itsexposure device for writing image.
 4. An optical scanning devicecomprising a light deflective optical system in which two stationaryplane mirrors are disposed to face a deflective reflecting facet whichis parallel to a rotational axis and can be rotated or swivelled aboutthe rotational axis such that a light beam being incident on andreflected from said deflective reflecting facet is reflected by said twostationary plane mirrors sequentially and is incident on and reflectedby the deflective reflecting facet again, wherein assuming that a planebeing parallel to said rotational axis and including the light beam tobe first incident on said deflective reflecting facet is an incidentplane, said two stationary plane mirrors are disposed perpendicular tosaid incident plane, and said optical scanning device is structured tosatisfy the following relation:0.33·θ1-1.27≦θ2≦0.35·θ1-1.50  (22) where θ1 is the incident angle of thecentral ray of the light beam to be first incident on said deflectivereflecting facet at a position where said deflective reflecting facet isperpendicular to the said incident plane and θ2 is the exit angle of thecentral ray of the emergent light beam after the second reflection bysaid deflective reflecting facet.
 5. An optical scanning device asclaimed in claim 4, wherein said optical scanning device is structuredsuch that the light beam to be first incident on said deflectivereflecting facet passes through a space between said two stationaryplane mirrors and the deflected light beam after the second reflectionby said deflective reflecting facet passes through the space betweensaid two stationary plane mirrors.
 6. An optical scanning device asclaimed in claim 5, wherein said optical scanning device is structuredsuch that the one of said two stationary plane mirrors is positioned tobe sandwiched between the light beam first incident on said deflectivereflecting facet and the emergent light beam after the second reflectionby said deflective reflecting facet.
 7. An optical scanning devicecomprising a light deflective optical system in which two stationaryplane mirrors are disposed to face a deflective reflecting facet whichis parallel to a rotational axis and can be rotated or swivelled aboutthe rotational axis such that a light beam being incident on andreflected from said deflective reflecting facet is reflected by said twostationary plane mirrors sequentially and is incident on and reflectedby the deflective reflecting facet again, wherein assuming that a planebeing parallel to said rotational axis and including the light beam tobe first incident on said deflective reflecting facet is an incidentplane, said two stationary plane mirrors are disposed perpendicular tosaid incident plane, and the central ray of an emergent light beam whenthe emergent light beam after the second reflection by said deflectivereflecting facet is on said incident plane and a straight line as thecentral ray of an emergent light beam being projected on the incidentplane when the deflective reflecting facet is revolved by the maximumrotational angle intersect with each other at a position near at leastone of optical surfaces disposed in the optical axial direction of ascanning optical system between said deflective reflecting facet and animaging surface.
 8. An optical scanning device as claimed in claim 7,wherein said optical scanning device is structured such that the lightbeam to be first incident on said deflective reflecting facet passesthrough a space between said two stationary plane mirrors and thedeflected light beam after the second reflection by said deflectivereflecting facet passes through the space between said two stationaryplane mirrors.
 9. An optical scanning device as claimed in claim 7,wherein said optical scanning device is structured such that the one ofsaid two stationary plane mirrors is positioned to be sandwiched betweenthe light beam first incident on said deflective reflecting facet andthe emergent light beam after the second reflection by said deflectivereflecting facet.
 10. An optical scanning device comprising: a lightdeflective optical system in which two stationary plane mirrors aredisposed to face a deflective reflecting facet which is parallel to arotational axis and can be rotated or swivelled about the rotationalaxis such that a light beam being incident on and reflected from saiddeflective reflecting facet is reflected by said two stationary planemirrors sequentially and is incident on and reflected by the deflectivereflecting facet again; an illumination optical system which irradiatesa light beam to said deflective reflecting facet; and a scanning opticalsystem which projects the light beam deflected by said light deflectiveoptical system to an imaging surface to form a scanning line, whereinassuming that a plane being parallel to said rotational axis andincluding the light beam to be first incident on said deflectivereflecting facet is an incident plane, said two stationary plane mirrorsare disposed perpendicular to said incident plane, and an emergent lightbeam when the emergent light beam after the second reflection by saiddeflective reflecting facet is on said incident plane has such arelation that the second reflection point and said imaging surface aresubstantially conjugated to each other on the incident plane, andwherein said optical scanning device is structured such that the lightbeam to be first incident on said deflective reflecting facet passesthrough a space between said two stationary plane mirrors and thedeflected light beam after the second reflection by said deflectivereflecting facet passes through the space between said two stationaryplane mirrors.
 11. An optical scanning device as claimed in claim 10,wherein when the emergent light beam after the second reflection by saiddeflective reflecting facet is on said incident plane, the light beamirradiated from said illumination optical system converges near thesecond reflection point on said incident plane.
 12. An optical scanningdevice as claimed in claim 11, wherein the following relation issatisfied:|β·Ld·tan(2ε)/cos(2ε)|≦0.25·LP  (30) where Ld is the transfer distancefrom the first deflection point to the second deflection point when theemergent light beam after the second reflection by said deflectivereflecting facet is on said incident plane, ε is the angle of a surfacetilt error in said deflective reflecting facet, β is the transversemagnification in the direction of said incident plane of the scanningoptical system, and LP is a scanning pitch in said imaging surface. 13.An optical scanning device as claimed in claim 11, wherein the followingrelation is satisfied:|β·Ld·tan(2ε)/cos(2ε)|≦0.125·LP  (31) where Ld is the transfer distancefrom the first deflection point to the second deflection point when theemergent light beam after the second reflection by said deflectivereflecting facet is on said incident plane, ε is the angle of a surfacetilt error in said deflective reflecting facet, β is the transversemagnification in the direction of said incident plane of the scanningoptical system, and LP is a scanning pitch in said imaging surface. 14.An image forming apparatus employing an optical scanning device asclaimed in claim 11 as its exposure device for writing image.
 15. Anoptical scanning device as claimed in claim 10, wherein the followingrelation is satisfied:|β·Ld·tan(2ε)/cos(2ε)|≦0.25·LP  (30) where Ld is the transfer distancefrom the first deflection point to the second deflection point when theemergent light beam after the second reflection by said deflectivereflecting facet is on said incident plane, ε is the angle of a surfacetilt error in said deflective reflecting facet, β is the transversemagnification in the direction of said incident plane of the scanningoptical system, and LP is a scanning pitch in said imaging surface. 16.An image forming apparatus employing an optical scanning device asclaimed in claim 15 as its exposure device for writing image.
 17. Anoptical scanning device as claimed in claim 10, wherein the followingrelation is satisfied:|β·Ld·tan(2ε)/cos(2ε)|≦0.125·LP  (31) where Ld is the transfer distancefrom the first deflection point to the second deflection point when theemergent light beam after the second reflection by said deflectivereflecting facet is on said incident plane, ε is the angle of a surfacetilt error in said deflective reflecting facet, β is the transversemagnification in the direction of said incident plane of the scanningoptical system, and LP is a scanning pitch in said imaging surface. 18.An image forming apparatus employing an optical scanning device asclaimed in claim 17 as its exposure device for writing image.
 19. Animage forming apparatus employing an optical scanning device as claimedin any one of claims 1, 2, 4-9 and 10 as its exposure device for writingimage.
 20. An optical scanning device comprising: a light deflectiveoptical system in which two stationary plane mirrors are disposed toface a deflective reflecting facet which is parallel to a rotationalaxis and can be rotated or swivelled about the rotational axis such thata light beam being incident on and reflected from said deflectivereflecting facet is reflected by said two stationary plane mirrorssequentially and is incident on and reflected by the deflectivereflecting facet again; an illumination optical system which irradiatesa light beam to said deflective reflecting facet; and a scanning opticalsystem which projects the light beam deflected by said light deflectiveoptical system to an imaging surface to form a scanning line, whereinassuming that a plane being parallel to said rotational axis andincluding the light beam to be first incident on said deflectivereflecting facet is an incident plane, said two stationary plane mirrorsare disposed perpendicular to said incident plane, and an emergent lightbeam when the emergent light beam after the second reflection by saiddeflective reflecting facet is on said incident plane has such arelation that the second reflection point and said imaging surface aresubstantially conjugated to each other on the incident plane, andwherein the following relation is satisfied;|β·Ld·tan(2ε)/cos(2ε)|≦0.25·LP  (30) where Ld is the transfer distancefrom the first deflection point to the second deflection point when theemergent light beam after the second reflection by said deflectivereflecting facet is on said incident plane, ε is the angle of a surfacetilt error in said deflective reflecting facet, β is the transversemagnification in the direction of said incident plane of the scanningoptical system, and LP is a scanning pitch in said imaging surface. 21.An optical scanning device comprising: a light deflective optical systemin which two stationary plane mirrors are disposed to face a deflectivereflecting facet which is parallel to a rotational axis and can berotated or swivelled about the rotational axis such that a light beambeing incident on and reflected from said deflective reflecting facet isreflected by said two stationary plane mirrors sequentially and isincident on and reflected by the deflective reflecting facet again; anillumination optical system which irradiates a light beam to saiddeflective reflecting facet; and a scanning optical system whichprojects the light beam deflected by said light deflective opticalsystem to an imaging surface to form a scanning line, wherein assumingthat a plane being parallel to said rotational axis and including thelight beam to be first incident on said deflective reflecting facet isan incident plane, said two stationary plane mirrors are disposedperpendicular to said incident plane, and an emergent light beam whenthe emergent light beam after the second reflection by said deflectivereflecting facet is on said incident plane has such a relation that thesecond reflection point and said imaging surface are substantiallyconjugated to each other on the incident plane, and wherein thefollowing relation is satisfied:|β·Ld·tan(2ε)/cos(2ε)|≦0.125·LP  (31) where Ld is the transfer distancefrom the first deflection point to the second deflection point when theemergent light beam after the second reflection by said deflectivereflecting facet is on said incident plane, ε is the angle of a surfacetilt error in said deflective reflecting facet, β is the transversemagnification in the direction of said incident plane of the scanningoptical system, and LP is a scanning pitch in said imaging surface.